1. Field of the Invention
The present invention relates broadly to the field of telecommunications. More particularly, the present invention relates to a modem equalizer for a PCM modem where the modem equalizer will function properly in the presence of a robbed-bit signalling network.
2. State of the Art
With the ever-increasing importance of telecommunications for the transfer of data as well as voice, there has been a strong effort to increase data transfer rates over the telephone wires. Recently, the ITU-T adopted the V.34 Recommendation (International Telecommunication Union, Telecommunication Standardization Sector Recommendation V.34, Geneva, Switzerland 1994) which is hereby incorporated by reference herein in its entirety. The V.34 standard and subsequent amendments define modem operating speeds of 28.8 kbps up to 33.6 kbps, and the vast majority of modems being sold today adhere to the V.34 Recommendation. However, with the explosion in the use of the Internet, even at the V.34 transfer rates, downloading of large files available on the Internet can take long periods of time. Thus, recently, there has been a thrust to provide additional standards recommendations which will increase data transfer rates even further (note the TIA TR-30.1 PCM Modem ad hoc group and the ITU-T Study Group 16).
Recognizing that further increases in data rates is theoretically limited (see C. E. Shannon, "A Mathematical Theory of Communication," Bell System Technical Journal, 27:379-423, 623-656 (1948)), there have been various proposals to take advantage of the fact that much of the telecommunication network is now digital. For example, U.S. Pat. No. 5,394,437 to Ayanoglu et al., U.S. Pat. No. 5,406,583 to Dagdeviren, and U.S. Pat. No. 5,528,625 to Ayanoglu et al. (all assigned to AT&T/Lucent and all of which are hereby incorporated by reference herein in their entireties) all discuss techniques which utilize the recognition that the network is mostly digital in order to increase data transmission rates to 56 kbps and higher. Similarly, Kalet et al., "The Capacity of PAM Voiceband Channels," IEEE International Conference on Communications '93, pages 507-511 Geneva, Switzerland (1993) discusses such a system where the transmitting end selects precise analog levels and timing such that the analog to digital conversion which occurs in the central office may be achieved with no quantization error. PCT application number PCT/US95/15924 (Publication WO 96/18261) to Townshend (which is hereby incorporated by reference herein in its entirety) discusses similar techniques. All of the disclosures assume the use of PAM (pulse amplitude modulation) digital encoding technology rather than the QAM (quadrature amplitude modulation) currently used in the V.34 Recommendation. The primary difference between the AT&T technology and the Townshend reference is that the AT&T technology suggests exploiting the digital aspect of the telephone network in both "upstream" and "downstream" directions, while Townshend appears to be concerned with the downstream direction only. Thus, systems such as the ".times.2" technology of US Robotics which are ostensibly based on Townshend envision the use of the V.34 Recommendation technology for upstream communications. As will be appreciated by those skilled in the art, the technologies underlying the V.34 Recommendation, and the proposed 56 kbps modem are complex and typically require the use of high-end digital signal processors (DSPs).
One of the tasks of the modem is the task of equalizing incoming signals. Equalization is a technique used to compensate for distortion in analog signal lines. One of the distortions which is compensated for is Intersymbol Interference (ISI) which is described in more detail below. Prior to data communication, two communicating modems engage in a training sequence wherein the equalizers of the modem receivers are set to compensate for the quality of the analog signal line which links the modems. However, it has been found that training of the equalizer in the modem receiver is adversely affected if the connection between the receiving modem and the ultimate source of the received data includes both analog and digital signal lines. In particular, training is adversely affected by robbed bit signaling (RBS) introduced by the digital network into the digital data stream and by Digital Pad Attenuation (DPA) in the digital network.
RBS is a technique used in T1 network connections where the least significant bit of each nth data octet is replaced with a control bit by the network for control signalling. The frequency of robbed bits through a single T1 connection is one every sixth symbol. This "in-band" signalling is used to indicate things like "off-hook", "on-hook", "ringing", "busy signal", etc. RBS results in data impairment by changing transmitted bit values. When the data flowing through the network is digitized audio voice signals, a change in the least significant bit of some octets will introduce noise or distortion into the ultimate analog audio reproduction of the digitized voice signal. Nevertheless, this has been found to be acceptable for voice communications. When the data flowing through the network is digital data, however, RBS can have very serious consequences. For this reason, when data is sent through a T1 network connection, the least significant bit of each octet is often not used and the data is sent as seven bit symbols. This mitigates the RBS problem if the link between the two data units is completely digital, at the expense of reducing bandwidth of the link by one eighth. Since a T1 line carries twenty four 64 k channels, if one 64 k channel is used in a combined analog/digital link to a modem, conventional methods of eliminating RBS (i.e., using seven-bit symbols) would limit the maximum theoretical bandwidth of the connection to 56 k. This theoretical maximum would never be reached because of the presence of ISI and DPAs. Thus, the problem of RBS is a major obstacle to exploiting the maximum possible bandwidth of the combined analog and digital link; and if one wishes to exploit the entire bandwidth of the link, one must deal directly with the RBS problem.
The problem of RBS is even further complicated when the link between two data units includes several different digital legs such that the frequency of RBS is variable from one connection to another. In other words, if the link includes several different digital legs in which bits are robbed, the frequency of robbed bits can increase significantly.
During PCM modem training, it is impossible for the receiver to know whether a symbol has been impaired due to RBS. As a result, RBS can interfere with the equalizer training process and result in misadjustment of the equalizer. In turn, equalizer error will lead to improper and/or inadequate channel equalization, with the ensuing inability to perform other tasks such as channel measurements, etc. This will prevent the receiver from ever making proper compensations for ISI and DPAs.
Prior art FIG. 1, shows a simplified model of data transmission through a combined analog/digital link. At 11, binary data is provided in the digital part of the network and represents data from either an intrinsically binary source or an appropriately encoded (A-Law or .mu.-Law) analog quantity which is to be transmitted to the desired destination. The digital channel 13 carries binary data through one or more legs of the network. For purposes of FIG. 1, the digital channel can be characterized by the way it affects the binary data it carries relative to the original binary data presented at 11. The binary data carried by the digital channel 13 is at some point subjected to a digital-to-analog converter 15 which maps or translates binary data into an analog symbol format, such as a PCM level, suitable for transmission through an analog channel. The D/A conversion takes place at the rate of 1/T Hz, where T seconds is the duration of the symbol interval.
The analog signal is then provided to an analog channel 17 which carries the analog waveforms to a PCM modem 19. Due to its band-limited nature, the analog channel 17 introduces distortion or inter-symbol interference (ISI) as well as noise into the stream of analog waveforms it carries. ISI causes individual symbols to interact with one another, thereby distorting the signals.
The PCM modem 19 is coupled to the analog channel 17 and includes, among other things, a receiver 21 and an analog channel decoder 23. The receiver 21 receives the analog signals at its input, and includes means for synchronizing the receiver with the data source/transmitter. The receiver 21 also includes an equalizer which compensates for ISI distortion introduced in the analog channel. The equalized signal may then be translated by the analog channel decoder 23 and formatted into a binary data stream which is sent to a further destination 25 (e.g., a receiving computer).
As previously mentioned, the analog channel 17 introduces ISI which can significantly distort the signal at the receiver. Indeed, as a result of ISI, the symbol received at the receiver 21 at time instant kT is no longer determined by the transmitted symbol alone, but by a linear combination of a (theoretically possibly infinite) number of previously transmitted symbols. ISI can be a severe impairment for certain types of analog channels; especially those with spectral nulls in their magnitude response at various frequencies such as DC. The ISI problem of an analog channel is compounded by the fact that its impulse response is in general unknown. Thus, the ISI affects data symbols in an unknown way and seriously complicates the task of the receiver of correctly detecting the incoming symbols with a low probability of error. Noise added at the output of the analog channel compounds the difficulty of correcting for ISI.
Equalization of the signal to account for ISI and noise is absolutely essential for proper communications. Various equalizer filter architectures are known for such equalization. For example, an equalizer called a decision feedback equalizer (DFE) which is shown in prior art FIG. 2 is known to be effective in cases of severe amplitude distortion. As seen in FIG. 2, the DFE 30 includes a feed-forward finite impulse response (FIR) equalizer 32, first and second summers 34, 36, a decision block 38, and a feedback FIR equalizer 40. The feed-forward FIR provides a feedforward equalized component (Rff) to the summer 34 which is compared to the feedback equalized component (Rfb) provided by the feedback FIR 40 to provide an equalized estimated symbol (R). This estimated symbol (R) is provided to the decision block 38 as well as to the second summer 36. The decision block 38 generates a decision based on the equalized estimated symbol. The decision is fed back to the input of the feedback FIR equalizer 40, and is also sent to the second summer 36. The second summer 36 takes the difference between the estimated symbol and the output symbol, i.e., the error, and provides the error to the feed-forward FIR 32 and to the feedback FIR 40 in order to update the equalizer tap coefficients of the FIRs 32 and 40.
It should be appreciated that in the modem training period (prior to sending data), a decision block is not used. Rather, as seen in prior art FIG. 3, the decision block 38 is replaced with a reference generator 37 which generates a reference (known) sequence Tk. The reference sequence is compared at the second summer 36 to the estimated symbol R to provide an error e which is used as feedback in order to update the equalizer tap coefficients 31 (Cff) and 39 (Cfb) of the FIRs 32 and 40.
While the adaptive equalizer of FIGS. 2 and 3 is effective in compensating for ISI analog channel impairments, it does not account for digital channel impairments which can be present in PCM-type modem communications. In particular, the adaptive equalizers of the prior art are not effective in the presence of robbed bit signaling which causes octets in the training sequence to be translated into analog symbols which are different than what they would otherwise be. Similarly, octet transformation due to digital PAD attenuation is problematic in causing individual training symbols to be translated into different analog levels. The PAD impairment can occur with RBS, either before or after a bit is robbed.
It has been discovered that RBS, when present, occurs in a repeating frame of six bytes. Table 1, below, shows an example of an RBS Frame in which bytes 1, 3, and 4 are affected by RBS. It should be noted that, as mentioned above, RBS may be present in conjunction with DPAs and the robbed bit may occur before or after the pad.
TABLE 1 ______________________________________ ##STR1## ______________________________________